#!/usr/bin/env python
#-*- coding: utf-8 -*-
#__author__:vincentlc
#time: 16/4/3 : 19:44

'''
Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

According to the definition of LCA on Wikipedia:
“The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants
(where we allow a node to be a descendant of itself).”

        _______6______
       /              \
    ___2__          ___8__
   /      \        /      \
   0      _4       7       9
         /  \
         3   5
For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2,
since a node can be a descendant of itself according to the LCA definition.
'''

# Definition for a binary tree node.
class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution(object):
    def lowestCommonAncestor(self, root, p, q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
    def lowestCommonAncestor(self, root, p, q):
        if (root is None or p is None or q is None):
            return None

        ancestor = root

        while True :
            if (p.val < ancestor.val and q.val < ancestor.val):
                ancestor = ancestor.left
            elif (p.val > ancestor.val and q.val > ancestor.val):
                ancestor = ancestor.right
            else:
                return ancestor

